This invention relates in general to a method and apparatus for transducing and monitoring the performance of generator units, which may be DC or AC generators, either single phase or polyphase, as well as for controlling the loads connected thereto.
While the general principles and teachings herein are applicable to all types of generators, the invention will be described in detail for use with 3-phase power generators that are widely used in small electric power plants, as in hospitals, offshore platforms, ships, etc., and in remote areas where commercial power is not available, or in emergency situations when commercial power is temporarily disrupted.
In use, such AC power generators are subjected to frequent large power surges, voltage and current overloads, and fluctuations in the Dower factor, as machinery is switched ON or OFF.
Also, because two or more generators are normally connected in parallel, when one generator's prime mover unexpectedly shuts down, all of the small plant's electrical loads, and the motoring effects of the failing generator, will be suddenly transferred to the surviving generators. This is known as the "reverse power" condition, as distinguished from the "normal" or "forward" power condition, when all the generators produce electricity.
A reverse power condition often produces a steep load increase on the surviving generator, say 50% of its rated load, resulting in a shutdown within 500 ms due to either under speed or over temperature of the engine driving the surviving generator. Other frequently encountered and potentially harmful conditions to all systems connected to the power bus, include under or over voltage, under or over frequency, and over current with under frequency (fault current) at the same time.
While this invention has wide applications for military as well as civilian users, it will be described for use by the U.S. Defense Department on ships, such as destroyers, battleships, aircraft carriers, etc., primarily because that use is well documented, regulated and critical.
Such military applications have stringent requirements, as specified in military spec MIL-M-2435OB(SH) of Jul. 31, 1989, as subsequently amended, (hereinafter "the MIL spec").
This MIL spec requires that reverse power monitoring, such as a reverse power relay (RPR), when mounted in a generator control unit, sense a power reversal on a 60 Hz, 3-phase generator in time to trip a circuit breaker for removing the motorized generator from the power bus to prevent overloading the surviving generator's prime mover.
The Mil spec also requires that the over-power monitor, such as an over-power relay (OPR), sense a high power demand and provide, if required, two or more successive, critically-timed load sheddings by tripping selected load circuit breakers for the purpose of reducing the temporary excessive high power demand, thereby protecting both the generator that is carrying the load and its prime mover.
The current sensing signals used by OPRs and RPR's are supplied by current transformers (CT) which see usually only 80% of CT rating at full generator rated output. Examples are 5000/5 ratio CTs used for 2500 kW 4000 amp generators, and 4000/5 ratio CTs used for 2000 kW 3200 amp generators. The CTs are used to drive switchboard 0-5 amp meter movements and remote panel meters which allow for reading over-current conditions rather than "peg out" at generator rating.
Additionally, a different type of RPR must be used depending on the type of prime mover involved, i.e., Diesel, steam turbine, or gas turbine.
These requirements make it difficult to adjust the ship generators trip levels, since each type of generator requires different types of OPRs and RPRs, and different types of reverse-power metering and trip timing intervals.
This great diversity of requirements was met in the prior art by providing a wide range of transducers, meters, test equipments, and power monitor instruments. Hence, the U.S. Defense Department had to maintain, at strategic locations around the world, costly inventory to meet the diverse needs of its different ship classes. In addition, skilled technicians on military ships had to periodically test their transducers, meters and power monitor instruments against standards available only in a shipyard or port.
It is a general object of this invention to use a digital monitor and controller for transducing and monitoring the performance of DC or AC generator units, as well as for controlling the loads connected thereto in small power plants especially on ships.